The present invention relates generally to a hermetic compressor assembly and, more particularly, to such a compressor assembly having high and low pressure regions within a sealed housing, wherein it is desired to minimize gas and oil leakage from the high pressure regions into the low pressure regions to improve compressor efficiency.
In general, prior art hermetic compressor assemblies comprise a housing which is hermetically sealed and within which is located a compressor mechanism including a crankcase. The present invention can be applied to a reciprocating piston compressor having a scotch yoke control mechanism. In such a compressor, the crankcase defines a plurality of radially disposed cylinders and a central suction cavity into which the cylinders open. A crankshaft is rotatably journalled in axially aligned bearing in the crankcase and includes an eccentric portion located in the suction cavity. Pistons reciprocable in the cylinders are operably coupled to the eccentric portion by means of a scotch yoke mechanism. The scotch yoke mechanism typically includes a slide block defining a coupling bearing in which the eccentric portion is journalled. Suction gas from the refrigeration system is provided directly to the suction cavity and is introduced within the cylinders by means of suction valves associated with the pistons. The gas refrigerant is then compressed within the cylinder and discharged into the interior of the housing to provide a pressurized, or high side, sealed housing.
In the aforementioned compressor assembly, a pressure differential is created between the high pressure region defined by the housing and the low pressure region defined by the suction cavity within the crankcase. In a typical compressor, a pressure differential between high and low pressure regions may be on the order of a 4 to 1 ratio. As a result of this pressure differential, several problems arise relating to leakage of gas and oil from high pressure regions to low pressure regions. The primary disadvantage of gas leakage from the high side housing to the suction cavity is that compressor operating efficiency is reduced as the refrigeration system is bypassed and no useful work is performed. Leakage of excessive amounts of oil into the suction cavity may result in damage to suction valves in the piston valve assembly.
A primary source of gas leakage from the high pressure housing into the low pressure suction cavity is the leakage occurring past the crankshaft where it is journalled in bearings in the crankcase. The cylindrical sleeve bearings supporting the crankshaft are exposed to high pressure and low pressure at opposite ends thereof. Consequently, gas leakage occurs which reduces compressor operating efficiency. Also, high flow leakage through the bearings makes it difficult to lubricate the bearings properly. Specifically, oil introduced at a single location along the circumference of the crankshaft or the bearing is blown into the crankcase suction cavity before it is evenly distributed for effective lubrication. Accordingly, dry spots are created along the shaft bearing surface, which do not receive proper lubrication and, therefore, do not experience a long operating life.
A primary source of oil leakage into the suction cavity is the oil introduced at the surface of the eccentric portion of the crankshaft to lubricate the eccentric as it is journalled within a bearing in the scotch yoke slide block. As is the practice in virtually all crankshaft connecting rod assemblies, oil ducts leading to the surface of the eccentric portion are located on the unloaded journalled portion. Accordingly, a slight clearance is created to allow oil to flow so as to provide adequate lubrication. However, in the case of the aforementioned compressor assembly having a pressurized housing, the oil delivered to the eccentric portion in the suction cavity is essentially at the higher discharge pressure. As a result, excessive amounts of oil and gas are introduced within the suction cavity, thereby resulting in a loss of compressor operating efficiency. Furthermore, damage may occur to the crankshaft bearings, particularly the upper bearing, if the oil supply from the lubrication system is diminished or depleted due to excessive oil leakage at the location of the eccentric portion.
The problems associated with a scotch yoke compressor, as described herein, have not been addressed by the prior art, as evidenced by the fact that high side scotch yoke compressors are not generally commercially available. In a low side housing design, either a pressure differential between the suction cavity and housing interior does not exist, or it is of much lesser magnitude. In such a design, oil used for lubricating the crankshaft bearings is prevented from freely entering the suction cavity by means of a thrust bearing between the end of the bearing and the counterweight on the shaft. This prevents excessive amounts of oil at a nominal oil pump pressure from entering the suction cavity.
With respect to prior art attempts to limit the amount of oil entering the suction cavity from the crankshaft eccentric and slide block assembly, the idea of locating the oil opening on the unloaded side of the eccentric is so engrained in the prior art teachings that very few alternative methods have been proposed. More importantly, the problem has not been as severe in the case of compressor assemblies wherein a high pressure differential between the housing and the suction cavity does not exist. Although a smaller oil delivery hole in the eccentric portion would limit oil flow, smaller holes will result in drill bit breakage which would certainly present a problem in a mass production manufacturing environment. Another alternative to limit the flow of cil into the suction cavity is to alter the oil pump of the lubrication system to produce a smaller head of oil available at the eccentric portion.
While it is necessary for the proper operation of a compressor assembly of the type herein described to permit some small amount of oil to leak into the suction cavity, the prior art has not adequately addressed the problem of limiting leakage of excessive gas and oil into the suction cavity of a high side compressor. More specifically, leakage of gas and oil from regions of high pressure to regions of low pressure for a compressor mechanism within a pressurized housing have not been adequately addressed by the prior art. Also, proper lubrication of crankshaft bearings in such compressors remains a problem.